Various piston/piston rod assemblies rely upon different methods of securing the piston to the piston rod to prevent the piston from becoming loose from the rod. One method is to construct the piston and rod of a single piece. However, such method of construction is costly both from a materials standpoint and from a time-consuming standpoint. The machining operation requires that a piece of stock be turned down to the diameter of the piston and that the stock material be further turned down (machined) to the diameter of the rod (two rods if the assembly is a double-acting piston/piston rod assembly). As can be appreciated, such machining is time consuming and expensive. Furthermore, a considerable amount of material must be machined off the stock piece to provide the two different diameters, e.g., piston and piston rod diameters. The loss of this material is costly.
Another less expensive method of producing a piston/piston rod assembly is to produce the rods and piston from separate size stock materials closely matching the desired size of the piston and rod so that less material will be wasted in the machining process. However, some means must be used in securing the piston on the piston rod. Typically, this involves the steps of externally threading an end of the piston rod and internally threading the piston member and then mating the two pieces. It has been found, however, that after considerable use, the piston tends to separate from the piston rod. One typical method of aiding in securing the threaded piston and rod together is to provide a layer of adhesive (such as LOCKTITE) to the threaded portions of the rod and piston. However, it has been found that these adhesives, when dry, tend to create small slivers which get into the system and act as an abrasive.
It has been further found that a typical threaded connection between the rod and piston requires that the rod be provided with a shoulder at the inner end of the rod and that the inner face of the piston be assembled in abutting relation against the should on the rod. Such structure makes it virtually impossible for the outer peripheral surfaces of the piston and piston rod to be parallel to one another and to the internal surface of the bore, since the threads on the ends of the rod and in the interior of the piston must be cut on angle. Because of this type of construction, a certain amount of cocking occurs. In an attempt to overcome this problem, it is necessary to apply a very high torque to the rod and piston when assembling the two members. Such high torque unnecessarily stresses both the rod and piston material and consequently lowers the strength of the material.